This invention relates to a recording medium reproducible by utilizing near-field light and a near-field optical probe for reproducing information recorded on such a recording medium and, more particularly to-a recording medium and near-field optical probe that enhances reproducing resolution for information recorded with density.
In recent years, remarkable development has been made in optical reproducing apparatuses (DVD players, etc.) for reproducing information on recording media by illuminating laser light. However, the information recording density has reached a limitation because of a presence of a diffraction limit of laser light. In an attempt to break through such diffraction limit, a proposal has been made on a near-field light reproducing apparatus using an optical head provided with a microscopic aperture having a diameter of less than a wavelength of laser light to be utilized in reproducing so that near-field light (including both near field and far field) produced at the microscopic aperture or on a surface of the recording medium can be utilized, thereby increasing reproducible information recording density.
Conventionally, the near-field microscopes using a probe (hereinafter referred to as a near-field optical probe) having a microscopic aperture as mentioned above as an apparatus utilizing near-field light have been utilized for observing sample microscopic surface textures. As one of schemes utilizing near-field light in the near-field microscopes, there is a scheme that the near-field optical probe microscopic aperture and the sample surface are approached in distance to nearly a diameter of the near-field optical probe microscopic aperture so that near-field light can be produced at the microscopic aperture by introducing propagation light through the near-field optical probe and toward the near-field optical probe microscopic aperture (illumination mode). In this case, scattering light caused by the interaction between the produced near-field light and the sample surface involving an intensity and phase reflecting a sample surface microscopic texture is detected by a scattering light detection system. Thus, high resolution of observation is made feasible that could not be achieved by the conventional optical microscopes.
There is another scheme of the near-field microscopes utilizing near-field light that propagation light is illuminated toward a sample to localize near-field light over the sample surface wherein a near-field optical probe microscopic aperture is approached to the sample surface to nearly a diameter of the near-field optical probe microscopic aperture (collection mode). In this case, scattering light cause by the interaction between the localized near-field light and the near-field optical probe microscopic aperture involving an intensity and phase reflecting a sample surface microscopic texture is guided to a scattering light detection system through the near-field optical probe microscopic aperture, thus achieving observation with high resolution.
As a near-field microscope, Japanese Patent Laid-open No. 174542/1995, for example, has been proposed disclosing a scanning near-field atomic force microscope. This scanning near-field atomic force microscope adopts as near-field optical probe an optical waveguide sharpened at a tip to perform probe access control and scanning control for the atomic force microscope (AFM) thereby enabling observation of sample surface topology and optical characteristics. FIG. 11 is a block diagram showing a schematic configuration of the scanning near-field atomic force microscope.
In FIG. 11, a scanning near-field atomic force microscope 80 has, above a probe 89, a laser light source 83, a focus lens 84, a mirror 85 and a photoelectric conversion element 86 vertically divided into two. The light emitted from the laser light source 83 is collected by the focus lens 84 onto a probe top surface 82 so that the light reflected thereon is guided to the photoelectric conversion element 86 via the mirror 85. Meanwhile, the light emitted from a light source 94 for light information measurement is illuminated through a collimate lens 95 to a backside of a recording medium 81 over a prism 92 having a slant face treated for total reflection. Then, the light is guided to the other end of the probe 89 (not-sharpened base) that is proximate to the recording medium 81 and introduced to the photoelectric conversion element 87.
The prism 92 and recording medium 81 are set up on a rough movement mechanism 97 and fine movement mechanism 96 movable in XYZ directions. The signal detected by the photoelectric conversion element 86 is sent to a servo mechanism 93. Based on the signal, the servo mechanism 93 controls the rough movement mechanism 97 and fine movement mechanism 96 so that the deflection on the probe 89 cannot exceed a prescribed value when approaching of the probe 89 to the recording medium 81 or reading out data. The servo mechanism 93 is connected with a computer 99 to control operation of the fine movement mechanism 96 in a planar directions and receive information about the recording medium from a control signal of the servo mechanism 93. Meanwhile, when applying modulation to the light of the light source 94 or providing vibration by a vibration mechanism 88 to between the probe 89 and the recording medium 81, the signal obtained in the photoelectric conversion element 87 is connected to an analog input interface of the computer 99 via a lock-in amplifier 98 to detect optical information in synchronism with planar action of the fine movement mechanism 96. When no modulation or the like is applied to the light source 94, the signal obtained in the photoelectric conversion element 87 is directly connected to the analog input interface of the computer 99 without being passed through the lock-in amplifier 98.
The above near-field optical information reproducing apparatus utilizes the near-field microscope technology and observation scheme, and can reproduce information densely recorded on a recording medium by utilizing near-field light.
However, where the recording medium is increased in recording density by arranging data marks as information units in a close relationship, when conducting reproducing with the recording medium there encounters difficulty for the near-field optical probe used in the conventional near-field optical information reproducing apparatus to individually recognize and detect adjacent ones of the data marks. This problem is explained hereinbelow on an example of a near-field optical probe of a near-field optical information reproducing apparatus for information reproducing on the collection mode. FIG. 12 shows a recording medium 100 arranged with data marks 101 to produce near-field light. Incidentally, FIG. 12 shows one part of the recording medium 100 wherein the dotted circle 102 signifies a position that a data mark is possible to provide.
In FIG. 12 the data marks 101 are different in optical transmittance or refractive index, for example, from a base member 103 of the recording medium 100. The difference in optical property enables recognition of the presence or absence of a data mark 101. That is, in the data mark 101 the near-field light produced on a surface of the recording medium 100 is different in intensity or the like from that of the base material 103, which realizes to reproduce information configured by the data mark 101. Here, the near-field light on the surface of the recording medium 100 is produced by illuminating incident light, such as laser light, at a backside (surface not having data marks) of the recording medium 100 under a condition of total reflection. Incidentally, recording onto the data mark 101 is possible to realize by a phase change recording method or the like in the currently-marketed rewritable recording mediums.
FIG. 13 shows a relationship between a sectional view of the recording medium 100 taken on line D-D′ in FIG. 12 and near-field light produced by the data marks 101. Meanwhile, in FIG. 13, a near-field optical probe 110 is arranged above the recording medium 100. The near-field optical probe 110 moves in a rightward direction in the figure as scanning directions to sequentially detect near-field light produced through the data marks 101 of the recording medium 100. For example, provided that in FIG. 13 the regions recorded with a data mark 101 (101a, 101b, 101c) are taken as “1” while those not recorded with a data mark 101 is as “0”, a signal will be reproduced as “01101” from left of the figure.
Accordingly, the amplitude of near-field light in positions fully close to the corresponding data marks 101a, 101b, 101c to “1” can be expressed rectangular, in an ideal case, as represented in a near-field light amplitude distribution of FIG. 13 (on medium surface). With respect to this, a near-field light amplitude distribution (at microscopic opening) of FIG. 13 shows a near-field light amplitude distribution of near-field light reaching a microscopic aperture 111 of the near-field optical probe 110, i.e. at a position that a given distance is provided between the data mark 101 and the near-field optical probe 110. Each data mark 101 has a spread smoothly attenuating left and right with a maximum point given on a center axis of the data mark.
Meanwhile, a near-field light intensity distribution (at microscopic aperture) of FIG. 13 illustrates a near-field light intensity distribution offered by the above near-field light amplitude distribution (at microscopic aperture). As is shown, near-field light produced through the adjacent data marks 101a and 101b at a position reaching the microscopic aperture 111 of the near-field optical probe 110 overlaps at respective foots of near-field light amplitude. This results in an obscured boundary between near-field light produced through the data mark 101a and near-field light produced through the data mark 101b, thus lowering resolution in reproducing. Thus, the data marks are difficult to separately recognize in the microscopic aperture 111 position of the near-field optical probe 110.
The near-field optical information reproducing apparatus is to ultimately detect a data mark 101 or reproduce information by guiding, into a near-field optical probe, scattering light (propagation light) obtained by scattering near-field light reaching the microscopic aperture 111 of the near-field optical probe 110. Consequently, the problem with data mark separation is not negligible. This problem might be avoided by providing full spacing between the data marks 101. This however decreases recording density on the recording medium, impairing high-density recording medium reproducing as a merit of near-field optical information reproducing apparatus.
Meanwhile, in illumination-mode information reproducing, in order to separately recognize individual data marks densely arranged on a recording medium, it is possible to decrease a localization range of near-field light caused at the microscopic aperture by reducing the size of the microscopic aperture of the near-field optical probe. However, a high level technology is required to make smaller microscopic aperture. There encounters a problem that a decreased localized range of near-field light is also decreased in intensity and hence difficult to detect.